Irradiation with ultraviolet light and gamma-rays increases the level of DNA topoisomerase II in nuclei of normal and xeroderma pigmentosum fibroblasts.

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Abstract
DNA topoisomerase II alpha was monitored with the monoclonal antibody Ki-S1 in human fibroblasts after irradiation of cells with 254-nm UV light and gamma-rays from a Cs-137 source. DNA topoisomerase II alpha was localized immunohistochemically as bright fluorescent dots in the karyoplasm. Investigated fibroblasts originated from normal human donors and a xeroderma pigmentosum patient (XP12BE). All cell lines showed a time and dose-dependent increase in DNA topoisomerase II alpha abundance after irradiation. The increase may reflect enhanced accessibility of the enzyme, enhanced gene expression or enhanced stabilization of mRNA or protein molecules. The effect was detectable as early as 1 h after irradiation at doses greater than or equal to 3 J/m(2) or greater than or equal to 3 Gy. It passed through a maximum and decreased within 18 h (UV light) or 6 h (gamma-rays). Except for the duration of the response, no principal differences were seen between the effects caused by UV light and those elicited by gamma-rays. The increase in enzyme levels might be part of the well-known DNA damage responses which operate in cell-protective or DNA-reparative pathways or may reflect initiation of apoptosis. DNA topoisomerase I was detected with a commercially available polyclonal antibody raised against human DNA topoisomerase I. In unirradiated cells, DNA topoisomerase I was found to be mainly concentrated in nucleoli. Irradiation with gamma-rays changed the staining pattern in that it caused a multitude of DNA topoisomerase I-rich centers to occur which may reflect sites of transcription of radiation-inducible genes.